Natural carbon dioxide in the air contains a small percentage of molecules containing radioactive carbon 14. All plants derive their carbon compounds from ambient carbon dioxide in the air. All animals including humans derive their carbon compounds directly or indirectly from plants: directly by consuming vegetable matter, and indirectly by consuming animal products such as meat, milk products and eggs.
This results in a certain percentage of carbon atoms in all living tissue being that of radioactive carbon 14. When the living organism dies, no more new carbon 14 is absorbed. The existing carbon 14 then decays away with an approximate half-life of 5,730 years. This means that for every 5,730 years that dead plant or animal matter or products derived from plant or animal matter age, their natural radioactivity due to carbon 14 is reduced by half. This process of natural carbon 14 decay is the basis of a method for determining the age of dead plant or animal tissue or products derived thereof. This method for determining the age of once living tissue is well known as "Carbon 14 Dating".
The initial concentration of carbon 14 in the carbon atoms that make up the carbon dioxide of the air comes from the interaction of high energy ionizing radiation from the sun or in the upper gaseous layers of the earth. Therefore, carbon 14 based carbon dioxide is always being naturally produced. Plants grown under ambient conditions contain a considerable amount of radioactivity. Animals including humans which consume these plants are also radioactive due to the carbon 14 absorbed. Carbon 14 is a beta particle emitter which, after emitting harmful beta radiation internally within the organism, changes to nitrogen 14 which is stable. This is particularly troublesome for carbon 14 within our strands of DNA. Not only does the opportunity exist for damage from beta particles and recoiling nitrogen 14 atoms resulting from this decay, but also the valence state of the carbon 14 atom has changed. Instead of being the valence for carbon which is +4 or -4, it is now changed to that of nitrogen which is +5 or -3. Even if the beta particle emitted cleanly travels out of the body without incident and the recoil of the resulting nitrogen 14 atom does not break any bonds, this change in valence number causes further damage.